1. Field of the Invention
The present invention generally relates to a coating composition and a coating system including a clear coat layer formed from the coating composition. More specifically, the present invention relates to a coating composition that enables threshold adhesion strength to be achieved between glass and a substrate bonded through the coating system.
2. Description of the Related Art
Bonding of stationary glass, such as a windshield, to a vehicle body during a vehicle assembly process presents many quality and safety issues to automotive manufacturers. For example, motor vehicle safety standards (MVSS), such as MVSS 212, require a threshold adhesion strength between glass and vehicle bodies for ensuring that the glass remains bonded to the vehicle body, especially in the event of a crash or rollover situation, to minimize the incidence of injury to passengers of the vehicle resulting from detachment of the glass from the vehicle body. In fact, in crash and roll-over situations, the glass may provide additional support to a roof of the vehicle and may thereby help to prevent injuries resulting from compaction of the roof of the vehicle. Therefore, the glass preferably remains bonded to the vehicle body.
During the vehicle assembly process, a coating system is formed on the vehicle body prior to the point in the process at which the glass is bonded to the vehicle body. The coating system typically includes a clear coat layer, a base coat layer, and other sub-clear coat layers that are known in the art. The glass is bonded to the coating system of the vehicle body via a sealant that is typically applied in the form of a bead to the clear coat layer. Thus, the adhesion strength between the glass and the vehicle body is dependent upon the interaction between the sealant and the clear coat layer within the coating system.
Many factors may contribute to the robustness of the bond between the glass and the vehicle body, including the formulation of the sealant, the formulation of the clear coat layer, base coat layer, and other sub-clear coat layer(s) within the coating system on the vehicle body and inter-layer adhesion between those layers, compatibility of the formulations used for the sealant, the clear coat layer, the base coat layer, and the sub-clear coat layer(s) within the coating system, film build of the clear coat layer, the base coat layer, and other sub-clear coat layer(s) within the coating system, and the ability of the clear coat layer, base coat layer, and other sub-clear coat layer(s) in the coating system to cure at lower than normal times and temperatures.
While numerous methods of achieving the threshold adhesion strength have been used in the past, satisfaction of the adhesion strength thresholds has generally been accompanied by detrimental effects to the aesthetic qualities (e.g. appearance) and/or physical properties of the coating system or has required manual application of a reactive primer or a tape masking between the clear coat layer and the sealant.
Coating compositions that are used to form the clear coat layers are generally formulated based on a desire to achieve certain aesthetic qualities and/or physical properties in the resulting coating system, such as acceptable appearance, durability, resistance to scratching and degradation due to UV light, environmental etching, etc. For example, crosslinkable carbamate-functional polymers have been found to provide particularly desirable properties to the coating system in terms of resistance to environmental etching. Such crosslinkable carbamate-functional polymers are known in the art.
One strategy that has been used in the past to maximize the adhesion strength between the glass and the vehicle bodies has been to modify the coating compositions used to form the clear coat layer or to add additional additives into the coating compositions to provide functional groups for reacting with the sealant. The coating compositions are sensitive to modification or addition of additives, which may negatively impact one or more of the aesthetic qualities of the resulting clear coat layer. The effect of the additives or modifications on the coating composition is unpredictable and varies depending upon the type of coating composition. Modification of the coating composition or addition of additives may also affect the ability of the coating composition to satisfy low temperature cure requirements, thereby possibly resulting in a higher incidence of off-specification coating systems.
With regard to the manual application of the reactive primer between the sealant and the clear coat layer, known reactive primers have reactive groups that react with the sealant and the clear coat layer, and much care must be taken to avoid application of the reactive primer onto the clear coat layer outside of specific regions on the vehicle body where the glass is to be bonded. Automated application of the reactive primer is generally not feasible, and laborers are typically required to apply the reactive primer, thus adding cost and time to the vehicle assembly process.
While it is well known to use crosslinking agents in the coating compositions, there are drawbacks associated with certain crosslinking agents that are known for use in the coating compositions used to form the clear coat layer and that are known to result in threshold adhesion strength between the glass and the vehicle bodies. Such crosslinking agents include aminoplasts having a high content of imino groups of greater than 10% based on a total number of reactive sites present in a melamine used to form the aminoplast prior to reaction. While the aminoplasts having the high content of amino groups may help coating systems to achieve threshold adhesion strength between glass and a substrate bonded through the coating systems, the aminoplasts having the high content of imino groups present difficulties with regard to balancing viscosity of the coating composition with VOC levels and solids levels within the coating composition.
In view of the foregoing, there is a desire within the automotive and coatings industries to provide solutions to the problems associated with bonding stationary glass to coating systems, especially coating systems having a clear coat layer formed from carbamate-functional resins. More specifically, there is a desire to achieve threshold adhesion strength, in accordance with MVSS standards, between the glass and the vehicle bodies while minimizing detrimental effects to the aesthetic qualities of the coating system and while minimizing problems associated with high viscosity of aminoplasts having a high content of imino groups of greater than 10%.